Microbial controls on phosphate and lanthanide distributions during granite weathering and soil formation

Both microbial and geochemical factors control the form, distribution, and abundance of lanthanides during weathering acid soil formation in a profile in the Bemboka Granodiorite from southern New South Wales, Australia. Duri ng the initial stages of weathering, hydrous Ce-bearing lanthanide and lant hanide-aluminum phosphates (including rhabdophane and florencite) crystalli ze on etched apatite surfaces throughout the profile. However, their fate d epends greatly on their location within the profile. In rocks weathered 5-6 m below the soil zone (the lower profile), secondary lanthanide phosphates persist long after all apatite has been dissolved. However, in rocks weath ered within 2 m of the soil zone (the upper profile), secondary phosphates are dissolved and lanthanides other than Ce are removed. Bacteria and funga l hyphae are localized on secondary phosphate surfaces, suggesting that rha bdophane and florencite are solubilized in the upper profile due to organic complexation of dissolved lanthanides and/or uptake of phosphate by cells. In the soils (defined by loss of granitic texture), secondary phosphates a re replaced by Ce-oxides. Lanthanides (other than Ce) removed in solution f rom the upper profile precipitate as Ce-poor phosphates in the lower profil e when the solubility product is exceeded due to high dissolved phosphate c oncentrations in proximity to apatite. Thus, the upper profile is the sourc e of lanthanides added to the lower profile. In both the lower and upper we athered granite, the degree to which weight-based Ce abundances increase wi th increasing weathering is consistent with Ce immobility on the centimeter scale. We attribute the very high weight-based Ce abundances (up to 12 X c oncentrations in fresh rock) to extensive leaching and compaction during tr ansformation of weathered rock to soil. (C) 2000 Elsevier Science B.V. All rights reserved.